1. Field of the Invention
The present invention relates to an inverter circuit for lighting a backlight of a liquid crystal display (LCD), and a method for driving the same. Particularly, the present invention relates to a separately-excited inverter circuit for lighting a backlight, and a method for driving the same.
2. Description of the Related Art
For lighting the backlight of a liquid crystal display, a separately-excited inverter circuit has been used, which utilizes resonance of a secondary circuit of a transformer. Compared to a self-excited inverter circuit, the separately-excited inverter circuit realizes a brighter backlight at the same input power, and less heat generation by the backlight. Moreover, the separately-excited inverter circuit realizes a smaller transformer, thus obtaining an advantage that the inverter circuit can be miniaturized.
Japanese Patent Laid-open Nos. Hei 11 (1999)-297485, Hei 10 (1998)-199690, and the like disclose examples of such a separately-excited inverter circuit for lighting the backlight of a liquid crystal display. FIG. 1 shows a configuration example of an inverter circuit for a backlight, which is disclosed in Japanese Patent Laid-open No. Hei 11 (1999)-297485. In FIG. 1, a fluorescent tube 101 is a backlight fluorescent tube, which is provided on the back of an unillustrated liquid crystal display panel, and irradiates the liquid crystal display panel from its back. A circuit in a portion surrounded by a broken line indicated by reference numeral 100 is a separately-excited inverter circuit, which is used for lighting the fluorescent tube 101. In this inverter circuit 100, a resonant capacitor C11 is connected to a primary winding N1 of a transformer T1 in parallel therewith. Both ends of the resonant capacitor C11 are connected to collectors of transistors Q11 and Q12, respectively. Emitters of the transistors Q11 and Q12 are grounded. In addition, bases of the transistors Q11 and Q12 are connected to output terminals (not shown) of a drive pulse generator 102, respectively, through input terminals (not shown).
A secondary winding N2 of the transformer T1 is connected to the fluorescent tube 101. Note that one terminal of the secondary winding N2 of the transformer T1 is connected to one electrode terminal of the fluorescent tube 101 through a current-limiting capacitor C12. In this inverter circuit 100, the transistor Q11 and the transistor Q12 are alternately turned on and off according to a drive pulse P1 and a drive pulse P2 from the drive pulse generator 102. Thus, a boosted alternating voltage is generated in the secondary winding N2 of the transformer T1, this alternating voltage causes a current to flow into the fluorescent tube 101 through the current-limiting capacitor C12, and the fluorescent tube 101 is lighted.
In the case of the above-described conventional separately-excited inverter circuit for the backlight of a liquid crystal display, a resonant circuit is formed of a secondary leakage inductance of a transformer, a resonant capacitor, and a stray capacitance of the backlight. The stray capacitance of the backlight has different values between the time when the lighting of the backlight is begun and the time after the lighting becomes stable. A resonance frequency of the resonant circuit also has different values between the time when the lighting of the backlight is begun and the time after the lighting becomes stable.
In terms of lighting characteristics of the backlight, it is preferable to drive the backlight at around the resonance frequency when the lighting is begun. Meanwhile, in terms of a brightness efficiency of the backlight, it is preferable to drive the backlight at the resonance frequency after the lighting has become stable.
The inverter circuit for the backlight is often operated at a single drive frequency. With the emphasis on the brightness efficiency of the backlight, if the backlight is operated at the resonance frequency after the lighting has become stable, there may arise a problem that the backlight is not lighted, and the like. Moreover, with the emphasis on the lighting characteristics of the backlight, if the backlight is driven at the resonance frequency when the lighting is begun, there arises a problem that the brightness efficiency of the backlight is lowered.
In recent years, along with realization of a larger-sized liquid crystal display, a cold-cathode tube of a backlight used therein has become longer. The longer the cold-cathode tube becomes, the more likely a problem that the backlight is not lighted occurs if the backlight is operated at the resonance frequency after the lighting has become stable. When the cold-cathode tube is long, the difference of the resonance frequency of the resonant circuit in the separately-excited inverter circuit becomes large between the time when the lighting is begun and the time after the lighting has become stable. Moreover, a voltage at which lighting of the cold-cathode tube becomes stable also gets higher. Thus, in a case where a long cold-cathode tube is used, it is required to previously set a power-supply voltage to be sufficiently high. In a case where the long cold-cathode tube is used at a single drive frequency, the cold-cathode tube may not be lighted unless there is a sufficient margin in the power-supply voltage in consideration for the inductance of the transformer, the resonant capacitor, and a variation in the stray capacitance.